Although it is well known that Coronal Mass Ejections (CMEs) and their associated interplanetary shocks are responsible for the most severe geomagnetic storms and for the largest solar energetic particle events, we currently know very little about their three-dimensional structure and the fundamental processes that control them. The lack of a fundamental understanding of how CMEs work limits our ability to predict their consequences with any reliability. The Solar Sentinel mission would address this problem by providing in-situ measurements of the three dimensional structure of CMEs directed toward Earth from a vantage point well upstream of Earth.
The Solar Sentinel Mission would station three small spacecraft carrying plasma, magnetic field, and energetic-particle instruments in Earth-synchronous orbit, close to the Earth-Sun line, but well inside 1 AU. The spacecraft would be powered by solar sails that would allow them to: 1) navigate inward from their deployment point at L1; 2) maintain their stations upstream of Earth with a 1-year orbit; and 3) gradually vary their mutual separation to investigate three-dimensional structures. The spacecraft would be stationed forward of the L1 LaGrangian point with a mutual separation of ~0.1 AU, forming a triangle about the Earth-Sun line. An Earth-Sentinel distance of ~0.05 AU is about the furthest that can be achieved with modern sail materials.
The Solar Sentinel mission would determine the three-dimensional structure of solar wind disturbances before they strike the Earth, improving our ability to predict space weather. Most importantly, this mission would allow the determination of the magnetic and velocity structure of CMEs, enabling an understanding of the fundamental processes that control them, and a characterization of the properties of those CMEs that cause the largest geomagnetic storms. When complemented by measurements from missions like ACE and WIND closer to 1 AU Solar Sentinel can also investigate how these structures evolve as they convect outward from the Sun. By relating the plasma and magnetic field signatures of CMEs to simultaneous measurements of energetic particles, it should be possible to determine when and how these structures accelerate particles efficiently.
The location of Solar Sentinel stations ~0.05 AU upstream from Earth is ideal for providing greatly improved measurements of interplanetary space weather ~4 hours before solar wind structures impact the Earth. Such warning is sufficient to mitigate risks to ground- based power systems, and to warn the Space Station and other space systems of intense particle fluxes that sometimes are accelerated by CME driven shocks.